Doppler radar has been utilized to monitor the vital signs of a biological subject (e.g. respiration and heartbeat) for many years because of the advantage of non-contact. The Doppler radar transmits a wireless signal to the biological subject and then receives a reflection signal from the biological subject, wherein the reflection signal contains the Doppler phase shifts due to the vital signs of the biological subject, and the vital signs of the biological subject can be extracted by analyzing the reflection signal. However, the random body movements of the biological subject also cause the Doppler phase shifts in the reflection signal, making the extraction of the vital signs of the biological subject difficult because any slight movements of the body produce large-magnitude Doppler phase shifts that overwhelm those associated with the vital signs. The application of the Doppler radar to vital sign detection is therefore limited, and the elimination of the Doppler phase shifts resulting from the random body movements has become important recently for the Doppler radar vital sign detection system.
Please refer to the patents U.S. Pat. Nos. 8,721,554 and 8,754,772 for the prior art that uses two radar devices on the opposite sides of a human subject in a vital sign detection system. The subject's tiny movements due to vital signs relative to the two radar devices are in the same direction and thus the resultant Doppler phase shifts detected by the two radar devices are in phase. In contrast, the subject's body movements relative to the two radar devices are in opposite directions, and therefore the resultant Doppler phase shifts detected by the two radar devices are out of phase. According to the above characteristic, the Doppler phase shifts due to body movements can be cancelled out in the baseband (U.S. Pat. No. 8,721,554) or the RF front-end (U.S. Pat. No. 8,754,772) of the system, while the Doppler phase shifts due to vital signs can be preserved. However, the use of two radar devices increases the complexity of the system and its power consumption. Moreover, the system can tolerate only small body movements because large body movements commonly generate the nonlinear distortions and so cause great difficulty in extracting the vital signs. In a word, the prior art is difficult to monitor the vital signs of people who make large body movements.